Structures of withanolides. (A) WA falls under the family of compounds known as withanolides which are a group of naturally occurring C28- steroidal lactones built on an intact or rearranged ergostane framework, in which C-22 and C- 26 are appropriately oxidized to form a six-membered lactone ring. The basic skeleton shown here is designated as the withanolide skeleton defined as a 22-hydroxyergostan-26-oic acid-26,22-lactone. (B) Structure of WA. It contains two sites which are prone to nucleophilic attack: A six membered δ-valero lactone ring containing a carbocyclic ester group and a three membered epoxy ring.